Semiconductor chips or dies are typically encapsulated in a package that protects the chips from the surrounding environment. The packages typically include leads or other connection points that allow the encapsulated chip to be electrically coupled to another microelectronic component. Leaded packages include a semiconductor chip bonded to a lead frame either seated on a die paddle or attached directly to the leads in a leads-over-chip attachment. The contact pads on the semiconductor die are then electrically connected to the chip, e.g., by wire bonding. The connected lead frame and chip may then be encapsulated in a mold compound to form a complete microelectronic component package. In most common applications, the leads extend out from the mold compound, allowing the chip to be electrically accessed. Typically, the leads extend laterally outwardly in a flat array that is part of a lead frame. This lead frame may be trimmed and formed into a desired configuration.
One increasingly popular technique for maximizing device density on a substrate is to stack microelectronic devices on top of one another. Stacking just one device on top of a lower device can effectively double the circuitry within a given footprint; stacking additional devices can further increase the circuit density.
In one approach, multiple microelectronic components are assembled in a single package. FIG. 1 schematically illustrates a thin small outline package (TSOP) 10 that includes an upper microelectronic component 20 and a lower microelectronic component 30. Such a TSOP 10 may be used in a memory module for a microelectronic device, for example. Typically, these microelectronic components 20 and 30 are semiconductor dies. Leads 42 of an upper lead frame 40 may be physically attached to the upper microelectronic component 20 via an adhesive, such as a conventional lead-on-chip tape 22. The inner lengths 44 of some or all of the leads 42 are electrically coupled to the upper microelectronic component 20 by individual wire bonds 24. Similarly, leads 52 of a lower lead frame 50 are physically attached to the lower microelectronic component 30 by an adhesive 32. Wire bonds 34 electrically connect the inner lengths 54 of selected leads 52 to the lower microelectronic component 30. The upper microelectronic component 20 and the lower microelectronic component 30 may be attached in a variety of ways, such as by a die attach adhesive 25.
The microelectronic components 20 and 30 and the inner lengths 44 and 54 of the leads 42 and 52, respectively, may be encapsulated in a mold compound 12. An outer length 46 of each lead 42 of the upper lead frame 40 extends outwardly beyond a periphery 14 of the mold compound 12. Similarly, an outer length 56 of each lead 52 of the lower lead frame 50 extends outwardly beyond the periphery 14 of the mold compound 12. The outer lengths 56 of the lower leads 52 may be shaped for connection to a substrate 60 or another microelectronic component. The TSOP 10 shown in FIG. 1 employs lower leads 52 with generally S-shaped outer lengths, which is commonplace for TSOPs; a wide variety of other shapes are known in the art for use in different applications.
The TSOP 10 shown in FIG. 1, and a number of like devices, has little or no good way to dissipate heat generated by the microelectronic components 20 and 30. Air may flow about the exterior of the mold compound 12. In addition, the exposed outer lengths 46 and 56 of the leads 42 and 52, respectively, can help conduct heat away from the microelectronic components 20 and 30 to the ambient environment of the TSOP 10. However, the microelectronic components 20 and 30 themselves are encapsulated in the mold compound 12 and have little direct thermal communication with the ambient atmosphere. This hampers the ability to cool the microelectronic components 20 and 30, increasing the likelihood of failure of the microelectronic components 20 and 30 over time.
The mold compound 12 of such a TSOP 10 often has a coefficient of thermal expansion (CTE) that differs from the CTE of the leads 42 and 52, wire bonds 24 and 34, and microelectronic components 20 and 30. The changes in temperature inherent in conventional manufacturing processes can warp or otherwise damage the microelectronic components 20 and 30 and/or adversely affect the electrical connections between the microelectronic components 20 and 30 and the lead frames 40 and 50, respectively. If either one of the microelectronic components 20 and 30 or their respective electrical connections is damaged, the entire TSOP 10 is considered defective. As a consequence, either an otherwise acceptable microelectronic component must be discarded with the defective microelectronic component or the microelectronic components 20 and 30 must be separated from one another to remove the defective component. For highly cost-competitive products, such as memory modules, both of these options may prove unduly expensive.